1. Field of the Invention
The present invention relates in general to a voltage regulator, and more particularly to an improvement in responsibility of the voltage regulator and a stable operation of the voltage regulator.
2. Description of the Related Art
FIG. 4 is a circuit diagram of a conventional voltage regulator.
The voltage regulator includes a reference voltage circuit 10 for generating a reference voltage, bleeder resistors 11 and 12 with which an output voltage Vout of the voltage regulator is divided, a differential amplifier 20 for amplifying a difference between the reference voltage and a voltage appearing at a node between the bleeder resistors 11 and 12, and an output transistor 14 which is controlled in accordance with an output voltage of the differential amplifier 20.
When the output (reference) voltage of the reference voltage circuit 10 is assigned Vref, the voltage at the node between the bleeder resistors 11 and 12 is assigned Va, and the output voltage of the differential amplifier 20 is assigned Verr, if a relationship of Vref>Va is established, the output voltage Verr becomes low, while if a relationship of Vref≦Va is established, the output voltage Verr becomes high. When the output voltage Verr is low, since a gate to source voltage of the output transistor 14 is high and thus an ON resistance of the output transistor 14 becomes small, the output transistor 14 operates so as to increase the output voltage Vout. On the other hand, when the output voltage Verr is high, since the ON resistance of the output transistor 14 becomes large, the output transistor 14 operates so as to decrease the output voltage Vout. As a result, the output voltage Vout is held at a constant value.
In the case of the conventional voltage regulator, since the differential amplifier 20 is an amplifier circuit in a first stage, and a circuit constituted by the output transistor 14 and a load resistor 25 is an amplifier circuit in a second stage, a configuration of two-stage voltage amplification circuit is provided. A capacitor 22 for phase compensation is connected between the output of the differential amplifier 20 and a drain of the output transistor 14, and a frequency band of the differential amplifier 20 is narrowed by the mirror effect, thereby preventing the oscillation of the voltage regulator. As a result, the frequency band of the whole voltage regulator becomes narrow, and hence the responsibility of the voltage regulator becomes poor.
In general, when the responsibility of the voltage regulator is improved, it is necessary to widen the frequency band of the whole voltage regulator. However, when the frequency band of the whole voltage regulator is widened, it is necessary to increase a consumption current of the voltage amplifying circuit. In particular, when the voltage regulator is used for a battery of a portable device or the like, its operating time becomes shorter.
Also, when a three-stage voltage amplification is used, even if a consumption current is relatively small, the frequency band of the voltage regulator can be widened. However, because a phase is easily delayed by 180 degrees or more, the operation of the voltage regulator becomes unstable, which may cause oscillation thereof. Therefore, in the case of the three-stage voltage amplification, it is necessary to increase a capacitance value of the ceramic capacitor in order to reduce the phase at a zero point resulting from the load and an ESR (equivalent series resistance) of the capacitor.
[Patent Document 1] JP 4-195613 A (Page 3, FIG. 1)